1. Field of the Invention
The present invention relates to flat microtip display screens. The present invention more specifically relates to the manufacturing of such screens.
2. Discussion of the Related Art
A microtip screen is generally formed of a cathode provided with electron emission microtips placed facing an anode provided with phosphor elements likely to be energized by electron bombarding. The cathode is associated with a grid provided with holes corresponding to the locations of the microtips
The microtips are generally arranged on cathode conductors organized in columns and addressable individually. The grid is organized in rows perpendicular to the cathode columns, also addressable individually.
In a color screen, the anode is generally provided with alternate strips of phosphor elements each corresponding to a color (Red, Green, Blue). The strips are parallel to the cathode columns and are separated from one another by an insulator. The phosphor elements are deposited on electrodes formed of corresponding strips of a transparent conductive layer, for example, in indium and tin oxide (ITO).
The intersection of a cathode column and of a grid line defines a screen pixel. For a color screen, the sets of red, green, blue strips are alternately biased with respect to the cathode, so that the electrons extracted from the microtips of a pixel of the cathode/grid are alternately directed to each of the colors. In some color screens where the cathode columns (or the grid lines) are divided in three to correspond to each color, the intersection of a grid row with a cathode column then defines a sub-pixel of a color.
Generally, the grid rows are sequentially biased to a potential on the order of 80 volts, while the strips of phosphor elements to be energized are biased under a voltage on the order of 400 volts via the ITO strip on which the phosphor elements are deposited. The ITO strips, bearing the other strips of phosphor elements, are at a low or zero potential. The cathode columns are brought to respective potentials included between a maximum emission potential and a no emission potential (for example, respectively 0 and 30 volts). The brightness of a color component of each of the pixels in a line is thus determined.
In a monochrome screen, the anode is generally formed of a plane of simultaneously biased phosphor elements of same color, or of two sets of alternate strips of phosphor elements of same color addressed alternately.
The choice of the values of the biasing potentials is linked to the characteristics of the phosphor elements and of the microtips. Usually, below a potential difference of 50 volts between the cathode and the gate, there is no electron emission, and the maximum emission corresponds to a potential difference of 80 volts.
The manufacturing of microtip screens calls up techniques currently used in the manufacturing of integrated circuits. The cathode is generally formed of thin layer depositions on a substrate, for example, made of glass, forming the bottom of the screen. The anode is generally formed on a glass substrate forming the screen surface.
The anode and the cathode-grid are made independently from each other on both substrates, then are assembled by means of a peripheral seal while creating, between the grid and the anode, an empty space to enable the flowing of the electrons emitted by the cathode to the anode.
During assembly, the screen is submitted to various thermal degassing processings. These processings are generally performed under pumping by means of a tube communicating with the empty space and meant to be closed at the end of the manufacturing process.
A getter is generally introduced in the screen, for example, in the tube, before closing. This getter has the function of trapping elements desorbed, in particular by the anode, during screen operation. However, this getter is inactive for neutral species, especially rare gases, which remain in the empty space after closing of the screen.
A trapping of the species remaining in the space between electrodes must thus be caused to improve the vacuum. This ultimate step is performed once the pumping tube is closed. It consists of causing an electron emission by the microtips to ionize neutral species remaining in the space between electrodes. The bombarding of the neutral species causes an extraction of an electron from their valence layer and these species are then positively charged. They are then attracted by the microtips at the most negative potential. This step is generally called an ion pumping.
The present invention more specifically relates to the improvement of the vacuum of the space between electrodes by ion pumping.
A disadvantage of conventional screens is that the ion pumping damages the cathode microtips. Indeed, the collection of the ionized species by the microtips causes a mechanical and/or chemical erosion (especially by rare gases) of the microtips. Although the screen vacuum is improved, a decrease in the microtip emissivity is observed.
Another disadvantage of conventional screens is that, during screen operation, some outgassed species do not succeed in being trapped by the getter. This results in a decrease of the quality of the vacuum which is prejudicial to the screen reliability.